Growth hormone (GH) is produced by somatotropes of the anterior pituitary gland and is released into the general circulation where it stimulates the production of insulin-like growth factor I (IGF-I) from multiple target tissues. GH and IGF-I work in concert to mediate various aspect of cellular function. In normal individuals, circulating GH/IGF-I levels steadily rise after to birth, plateau around the peripubertal period, then decline thereafter at a rate of ~14% for every decade of life. Circulating GH levels also decline with weight gain, independent of age. Based on human and animal studies examining the metabolic impact of GH deficiency (GHD), with and without GH or IGF-I replacement, it is clear that GH can inhibit fat accumulation and promote protein accretion, while the metabolic actions of IGF-I resemble those of insulin. GH and IGF-I are also reported to have positive effects on pancreatic b-cell function, cardiovascular performance and bone remodeling. The metabolic benefits of GH replacement in GHD patients has fueled the off-label use and abuse of GH in normal healthy adults to enhance body image and athletic performance, as well as providing a rationale for GH replacement in elderly and/or obese individuals to help combat metabolic disease. Despite the potential positive effects of GH in adults, there is also ongoing debate whether GH, at physiologic levels, can reduce insulin sensitivity, exacerbate cancer, and shorten lifespan. The true importance of endogenous GH in maintaining health OR promoting disease in adults, remains to be clarified since the bulk of our knowledge is based on studies of 1 ) acute or long-term GH administration in normal subjects, 1) developmental GHD, that might not reflect the consequences of GH decline after sexual maturation and 2) adult onset GHD (AOGHD) which is frequently accompanied by other pituitary defects, making it difficult to determine what changes are due specifically to GH loss. In addition, in humans it is difficult to separate out the confounding effects of genetics, environment and lifestyle from those due specifically to endocrine pathologies. To circumvent these problems and more accurately define the importance of endogenous GH in adult (patho)physiology the current proposal will utilize a unique genetically engineered mouse model of adult-onset, isolated GHD (AOiGHD). Preliminary studies demonstrated when AOiGHD mice were fed a low-fat diet, adipose mass increased, but whole body insulin sensitivity improved. However, diet-induced obese AOiGHD mice developed glucose intolerance which was associated with low fed and fasted insulin levels, suggesting GH/IGF-I may also be important in maintaining b-cell function. Given multiple organs systems are involved in maintaining glucose homeostasis, the proposed series of studies will take an integrative approach to determine 1) at what level AOiGHD impacts b-cell function (insulin expression, proliferation, neogenesis and/or apoptosis), 2) if the AOiGHD-induced increase in insulin sensitivity is due to global improvement in insulin actions or due to tissue-specific changes (liver, fat, skeletal muscle) 3) determine if AOiGHD will influence whole body metabolism and weight changes in response to acute fasting/refeeding or longterm diet-induced weight loss. Knowledge gained from these studies will provide important information regarding the benefits or risks of GH replacement therapy.